The Lerderderg River (known locally as The Lerdy[1]) is a perennial river of the Port Phillip catchment, located in the Western District region of the Australian state of Victoria.

The river flows generally south by east in a highly meandering course, through the Lerderderg Gorge within the Wombat State Forest where it is joined by two minor tributaries, The Old River and Goodman Creek, before reaching its confluence with the Werribee River within the Melton Reservoir, southeast of Bacchus Marsh.

The river descends approximately 383 metres (1,257 ft) over its 40 kilometres (25 mi) course.

[4][5] The river is known for the Lerderderg Gorge Picnic Ground, camping, bushwalking, and fishing.

[citation needed] The river is traversed by the Western Freeway northeast of Bacchus Marsh.